The invention relates to a dual-clutch transmission for a vehicle and to a method for carrying out a pressure reduction mode in such a hydraulic system.
In a dual-clutch transmissions, fully automatic gear change is made possible by two sub-transmissions without tractive force interruption. Torque is transmitted via one of two clutches that connect the two sub-transmissions to the drive. This results in a gear change without tractive force interruption by closing one of the clutches, while at the same time the other opens.
The clutches and the shift elements for engaging the gears are typically hydraulically controlled. The hydraulic system required for this purpose operates in the driving mode at an accumulator pressure of, for example, in the order of 30 bar, which is provided by a pressure accumulator of the hydraulic system. Arranged in the pressure lines, routed to the hydraulic cylinders of both clutches and the gearshift elements, are control valves which are respectively electrically operated by a control unit. With the assistance of the control valves, hydraulic pressure applied to the first clutch and the second clutch can be adjusted. Associated to the control unit are moreover first and second pressure sensors which respectively monitor the hydraulic pressure applied to the first clutch and the second clutch. When, for example, the detected pressure sensor signals fail to correspond with the operating condition of the first/second clutch, the control unit is able to identify the presence of a faulty operation of the dual-clutch transmission.
In view of the very small gap dimensions of control valves in the hydraulics and as a result of the oil additives that are large in relation thereto, deposits can develop at the valve gaps, when the accumulator pressure is high and the vehicle is not in operation for an extended period (for example, overseas transport, vacation, etc.). These deposits adversely affect comfort/function. The complete emptying of the accumulator would be suitable as a countermeasure though.
For this reason, the pressure accumulator of the hydraulic system can be completely emptied, when detecting extended vehicle downtimes; This, however, is not sensible for energetic reasons. As an alternative, the accumulator pressure may be reduced to, for example, 10 bar. As a result, hydraulic oil deposits are prevented in the control valves, on the one hand. On the other hand, at least part of the stored pressure energy can still be used during pressure build-up for the next driving operation.